exec/memory: Expose memory_region_access_valid()

Instead of having hardware device poking into memory
internal API, expose memory_region_access_valid().

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20221217152454.96388-2-philmd@linaro.org>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>